Signal processing method in cochlear implant

ABSTRACT

A signal processing method in cochlear implant is performed by a speech processor and comprises a noise reduction stage and a signal compression stage. The noise reduction stage can efficiently reduce noise in a electrical speech signal of a normal speech. The signal compression stage can perform good signal compression to enhance signals to stimulate cochlear nerves of a hearing loss patient. The patient who uses a cochlear implant performing the signal processing method of the present invention can understand normal speech.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a signal processing method, and more particularly to a signal processing method in cochlear implant.

2. Description of Related Art

Cochlear implant is a surgically implanted electronic device that provides a sense of sound to hearing loss patients. The tremendous progress of the cochlear implant technologies has enabled many hearing loss patients to enjoy high level of speech understanding quality

Noise reduction and signal compression are critical stages in the cochlear implant. For example, a conventional cochlear implant comprising multiple microphones can enhance the sensed speech volume. However, noises in the sensed speech are also amplified and compressed so as to affect the speech understanding of the hearing loss patient. Besides, the multiple microphones increase hardware cost.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a signal processing method in cochlear implant. The signal processing method is performed by a speech processor and comprises a noise reduction stage and a signal compression stage.

The noise reduction stage comprises:

-   -   receiving a t-th noisy frame y_(t), wherein the t-th noisy frame         y_(t) is from an electrical speech signal y;     -   reducing noises in the t-th noisy frame y_(t) to obtain a t-th         clean frame x_(t); and     -   outputting the t-th clean frame x_(t).

The signal compression stage comprises:

-   -   receiving an amplitude envelope of the t-th clean frame x_(t);     -   compressing the t-th clean frame x_(t) to form a t-th output         frame

z _(t)=α_(t)×(x _(t) −x _(t))+ x _(t), wherein

-   -   x _(t) is a mean of the amplitude envelope of the t-th clean         frame x_(t);     -   α_(t) is a compression factor;     -   when the t-th output frame z_(t) is in a monitoring range         between an upper boundary and a lower boundary, α_(t)=α_(t-1)Δα₁         and Δα₁ is a positive value; and     -   when the t-th output frame z_(t) is beyond the monitoring range,         α_(t)=α_(t-1)+Δα₂ and Δα₂ is a negative value; and     -   outputting the t-th output frame z_(t).

Another objective of the present invention is to provide a signal processing method in cochlear implant. The signal processing method is performed by a speech processor having a noise reduction unit and a signal compressor. The signal compressor has a compression unit, a boundary calculation unit, and a compression-factor-providing unit. The signal processing method comprises a noise reduction stage and a signal compression stage.

The noise reduction stage is performed by the noise reduction unit and comprises:

-   -   receiving a t-th noisy frame y_(t), wherein the t-th noisy frame         y_(t) is from an electrical speech signal y;     -   reducing noises in the t-th noisy frame y_(t) to obtain a t-th         clean frame x_(t); and     -   outputting the t-th clean frame x_(t).

The signal compression stage is performed by the signal compressor and comprises:

-   -   receiving an amplitude envelope of the t-th clean frame x_(t) by         the compression unit and the boundary calculation unit;     -   calculating an upper boundary and a lower boundary and         transmitting the upper boundary and the lower boundary to the         compression-factor-providing unit by the boundary calculation         unit;     -   compressing the t-th clean frame x_(t) to obtain a t-th output         frame z_(t) and outputting the t-th output frame z_(t) by the         compression unit; and     -   calculating a compression factor α_(t) and transmitting the         compression factor α_(t) to the compression unit by the         compression-factor-providing unit according to the t-th output         frame z_(t), the upper boundary, and the lower boundary; wherein

z _(t)=α_(t)×(x _(t) −x _(t))+ x _(t); and

-   -   x _(t) is a mean of an amplitude envelope of x_(t), t=1, . . .         T,         where T is the lengths of the current utterance;     -   when the t-th output frame z_(t) is in a monitoring range         between the upper boundary and the lower boundary,         α_(t)=α_(t-1)+Δα₁ and Δα₁ is a positive value; and     -   when the t-th output frame z_(t) is beyond the monitoring range,         α_(t)=α_(t-1)+Δα₂ and Δα₂ is a negative value.

Based on the signal processing method of the present invention, the noise reduction stage can efficiently reduce noise in the electrical speech signal of the normal speech, and the signal compression stage can perform good signal compression to enhance signals to stimulate cochlear nerves of a hearing loss patient, such that the hearing loss patient can well understand the normal speech. The present invention performs the noise reduction stage and the signal compression stage to improve performance of the cochlear implant instead of using multiple microphones. Compared with the conventional cochlear implant with multiple microphones, the present invention would not increase the hardware cost.

Embodiments of the present invention are described in detail as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit block diagram of a cochlear implant;

FIG. 2 is a detailed circuit diagram including a speech processor connected to a microphone and pulse generators of the present invention;

FIG. 3 is a schematic view of a single-layered DAE-based NR structure;

FIG. 4(a) shows an amplitude envelope of a clean speech signal;

FIG. 4(b) shows an amplitude envelope of a noisy speech signal;

FIG. 4(c) shows an amplitude envelope detected by a conventional log-MMSE estimator;

FIG. 4(d) shows an amplitude envelope detected by a conventional KLT estimator;

FIG. 4(e) shows an amplitude envelope detected by the present invention;

FIG. 5 is a circuit block diagram of one channel of the speech processor of the present invention;

FIG. 6 is a waveform diagram of an amplitude envelope detected by an envelope detection unit; and

FIG. 7 is a waveform diagram of an output frame generated by the signal compressor of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, a basic and conventional configuration of a circuit block diagram of a cochlear implant comprises a microphone 11, a speech processor 12, a transmitter 13, a receiver 14, a pulse generator 15, and an electrode array 16. The microphone 11 and the speech processor 12 are assembled to be mounted on a patient's ear. The transmitter 13 is adapted to be attached on an exterior surface of the patient's head skin. The receiver 14, the pulse generator 15, and the electrode array 16 are implanted under the patient's head skin.

The microphone 11 is an acoustic-to-electric transducer that converts a normal speech in air into an electrical speech signal. The speech processor 12 receives the electrical speech signal and converts the electrical speech signal into multiple output sub-speech signals in different frequencies. The transmitter 13 receives the output sub-speech signals from the speech processor 12 and wirelessly sends the output sub-speech signals to the receiver 14. The pulse generator 15 receives the output sub-speech signals from the receiver 14 and generates different electrical pulses based on the output sub-speech signals to the electrode array 16. The electrode array 16 has multiple electrodes 161 respectively and electrically connected to different cochlear nerves of the hearing loss patient's inner ear. Therefore, the electrodes 161 respectively output the electrical pulses to stimulate the cochlear nerves, such that the patient can hear something approximating the normal speech.

In more details, with reference to FIG. 2, the speech processor 12 has multiple channels including a first channel, a second channel, . . . , an i-th channel, . . . , and a n-th channel, wherein i and n are positive integrals. Each one of the channels has a band-pass filter 121, an envelope detection unit 122, and a signal compressor 123. The envelope detection unit 122 is used to detect an amplitude envelope of a signal and can have a rectifier 124 and a low-pass filter 125. In the present invention, a noise reduction unit 126 is added. The noise reduction unit 126 is connected between the microphone 11 and the band-pass filters 121 of each one of the channels. In time domain, when the noise reduction unit 126 receives the electrical speech signal from the microphone 11, the noise reduction unit 126 segments the electrical speech signal into several continuous frames to reduce noise of the frames. For example, when a time length of the electrical speech signal is 3 seconds, the noise reduction unit 126 can segment the electrical speech signal into 300 continuous frames, wherein a time length of each one of the frames of the electrical speech signal is 10 milliseconds.

Based on the above configuration, the band-pass filter 121 of each one of the channels sequentially receives the frames of the electrical speech signal from the noise reduction unit 126. The band-pass filter 121 of each one of the channels can preserve elements of each one of the frames of the electrical speech signal within a specific frequency band and remove elements beyond the specific frequency band from such frame. The specific frequency bands of the band-pass filters 121 of the channels are different from each other. Afterwards, the amplitude envelopes of the frames of the electrical speech signal are detected by the envelope detection units 122 and provided to the signal compressors 123 respectively.

The present invention relates to a noise reduction stage performed by the noise reduction unit 126 and a signal compression stage performed by the signal compressor 123. The noise reduction stage and the signal compression stage are respectively described below.

1. Noise Reduction Stage

The noise reduction unit 126 can be performed in a DDAE (deep denoising autoencoder)-based NR (noise reduction) structure. The DDAE-based NR structure is widely used in building a deep neural architecture for robust feature extraction and classification. In brief, with reference to FIG. 3, a single-layered DAE-based NR structure comprises an input layer 21, a hidden layer 22, and an output layer 23. The DDAE-based NR structure is a multiple-layered DAE-based NR structure comprising the input layer 21, the output layer 23, and multiple hidden layers 22. Because the parameter estimation and speech enhancement procedure of DDAE is the same as those of single-layered DAE, we only present the parameter estimation and speech enhancement for the single-layered DAE for ease of explanation. The same parameter estimation and speech enhancement procedures can be followed for the DDAE.

The input layer 21 receives an electrical speech signal y from the microphone 11 and segments the electrical speech signal y into a first noisy frame y₁, a second noisy frame y₂, . . . , a t-th noisy frame y_(t), . . . , and a T-th noisy frame y_(T), wherein T is a length of the current utterance. For the elements in the t-th noisy frame y_(t), the noise reduction unit 126 reduces noise in the t-th noisy frame y_(t) to form a t-th clean frame x_(t). Afterwards, the output layer 23 sends the t-th clean frame x_(t) to the channels of the speech processor 12 respectively.

A relationship between the t-th noisy frame y_(t) and the t-th clean frame x_(t) can be represented as:

x _(t) =W ₂ h(y _(t))+b ₂  (1)

-   -   where     -   h(y_(t)) is a function including W₁ and b₁ in time domain;     -   W₁ and W₂ are default connection weights in time domain; and     -   b₁ and b₂ are default vectors of biases of the hidden layers 22         of the DDAE-based NR structure in time domain.

Besides, in another embodiment, the relationship between the t-th noisy frame y_(t) and the t-th clean frame x_(t) can be represented as

x _(t) =InvF{(W ₂ ′h′(F{y _(t)})+b ₂′)}  (2)

-   -   where     -   F { } is a Fourier transform function to transfer the t-th noisy         frame y_(t), from time domain to frequency domain;     -   h′( ) is a function including W₁′ and b₁′;     -   W₁′ and W₂′ are default connection weights in frequency domain;     -   b₁′ and b₂′ are default vectors of biases of the hidden layers         22 of the DDAE-based NR structure in frequency domain; and     -   InvF { } is an inverse Fourier transform function to obtain the         t-th clean frame x_(t).

According to experimental result, the t-th clean frame x_(t) deduced from the Fourier transform and the inverse-Fourier transform as mentioned above has better performance than which without the Fourier transform and the inverse-Fourier transform.

For the time domain based method as shown in equation (1), h(y_(t)) can be represented as:

$\begin{matrix} {{h\left( y_{t} \right)} = {{\sigma \left( {{W_{1}y_{t}} + b_{1}} \right)} = \frac{1}{1 + {\exp \left\lbrack {- \left( {{W_{1}y_{t}} + b_{1}} \right)} \right\rbrack}}}} & (3) \end{matrix}$

For the frequency domain based method as shown in equation (2), h′ (F {y_(t)}) can be represented as:

$\begin{matrix} {{h^{\prime}\left( {F\left\{ y_{t} \right\}} \right)} = {{\sigma \left( {{W_{1}^{\prime}F\left\{ y_{t} \right\}} + b_{1}^{\prime}} \right)} = \frac{1}{1 + {\exp \left\lbrack {- \left( {{W_{1}^{\prime}F\left\{ y_{t} \right\}} + b_{1}^{\prime}} \right)} \right\rbrack}}}} & (4) \end{matrix}$

Regarding the parameters including W₁, W₂, b₁, and b₂ in time domain or W₁′, W₂′, b₁′, and b₂′ in frequency domain, they are preset in the speech processor 12.

For example, in time domain, the parameters including W₁, W₂, b₁, and b₂ in equations (1) and (3) are obtained from a training stage. Training data including a clean speech sample u and a corresponding noisy speech sample v. Likewise, the clean speech sample u is segmented into several clean frames u₁, u₂, . . . , u_(T′), and the noisy speech sample v is segmented into several noisy frames v₁, v₂, . . . , v_(T′), wherein T′ is a lengths of a training utterance.

The parameters including W₁, W₂, b₁, and b₂ of equation (1) and equation (3) are optimized based on the following objective function:

$\begin{matrix} {\theta^{*} = {\arg \mspace{14mu} {\min_{\theta}\left( {{\frac{1}{T^{\prime}}{\sum_{t = 1}^{T^{\prime}}{{u_{t} - {\overset{\_}{u}}_{t}}}_{2}^{2}}} + {\eta \left( {{W_{1}}_{2}^{2} + {W_{2}}_{2}^{2}} \right)}} \right)}}} & (5) \end{matrix}$

In equation (5), θ is a parameter set {W₁, W₂, b₁, b₂}, T′ is a total number of the clean frames u₁, u₂, . . . , u_(T′), and η is a constant used to control the tradeoff between reconstruction accuracy and regularization on connection weights (for example, η can be set as 0.0002). The training data including the clean frames u₁, u₂, . . . , u_(T′) and the training parameters of W_(1-test), W_(2-test), b_(1-test) and b_(2-test) can be substituted into the equation (1) and equation (3) to obtain a reference frame ū_(t). When the training parameters of W_(1-test), W_(2-test), b_(1-test), and b_(2-test) can make the reference frame ū_(t), mostly approximate the clean frames u_(t), such training parameters of W_(1-test), W_(2-test), b_(1-test), and b_(2-test) are taken as the parameters of W₁, W₂, b₁, and b₂ of equation (1) and equation (3). Besides, when the noisy speech sample v approximates the electrical speech signal y, the training result of the parameters of W₁, W₂, b₁, and b₂ can be optimized. The optimization of equation (5) can be solved by using any unconstrained optimization algorithm. For example, a Hessian-free algorithm can be applied in the present invention.

After training, optimized parameters including W₁, W₂, b₁, and b₂ are obtained to be applied to equation (1) and equation (3) for real noise reduction application.

Besides, in frequency domain, the parameters including W₁′, W₂′, b₁′, and b₂′ of equation (2) and equation (4) are optimized based on the following objective function:

$\begin{matrix} {\theta^{*} = {\arg \mspace{14mu} {\min_{\theta}\left( {{\frac{1}{T^{\prime}}{\sum_{t = 1}^{T^{\prime}}{{u_{t} - {\overset{\_}{u}}_{t}}}_{2}^{2}}} + {\eta \left( {{W_{1}^{\prime}}_{2}^{2} + {W_{2}^{\prime}}_{2}^{2}} \right)}} \right)}}} & (6) \end{matrix}$

In equation (6), θ is a parameter set {W₁′, W₂′, b₁′, b₂′}, T′ is a total number of the clean frames u₁, u₂, . . . , u_(T′), and η is a constant used to control the tradeoff between reconstruction accuracy and regularization on connection weights (for example, η can be set as 0.0002). The training data including the clean frames u₁, u₂, . . . , u_(T′) and the training parameters of W_(1-test)′, W_(2-test)′, b_(1-test)′, and b_(2-test)′ can be substituted into the equation (2) and equation (4) to obtain a reference frame ū_(t). When the training parameters of W_(1-test)′, W_(2-test)′, b_(1-test)′, and b_(2-test)′ can make the reference frame ū_(t), mostly approximate the clean frames u_(t), such training parameters of W_(1-test)′, W_(2-test)′, b_(1-test)′, and b_(2-test)′ are taken as the parameters of W₁′, W₂′, b₁′, and b₂′ of equation (2) and equation (4). Besides, when the noisy speech sample v approximates the electrical speech signal y, the training result of the parameters of W₁′, W₂′, b₁′, and b₂′ can be optimized. The optimization of equation (6) can be solved by using any unconstrained optimization algorithm. For example, a Hessian-free algorithm can be applied in the present invention.

After training, optimized parameters including W₁′, W₂′, b₁′, and b₂′ are obtained to be applied to equation (2) and equation (4) for real noise reduction application.

With reference to FIGS. 4(a) and 4(b), FIG. 4(a) shows an amplitude envelope of a clean speech signal and FIG. 4(b) shows an amplitude envelope of a noisy speech signal. FIG. 4(c) shows an amplitude envelope detected by a conventional log-MMSE (minimum mean square error) estimator. FIG. 4(d) shows an amplitude envelope detected by a conventional KLT (Karhunen-Loeve transform) estimator. FIG. 4(e) shows an amplitude envelope detected by the present invention. Comparing FIG. 4(e) with FIG. 4(a), the detection result of the present invention is most approximate to the clean speech signal, which means the noise is removed. Comparing FIG. 4(b) with FIGS. 4(c) and 4(d), the detection results as illustrated in FIGS. 4(c) and 4(d) are still noisy.

According to experimental result as mentioned above, the signal performances of the conventional log-MMSE estimator and the KLT estimator are not as good as the present invention. The present invention has better noise reducing efficiency.

2. Signal Compression Stage

With reference to FIGS. 2 and 5, for the i-th channel of the speech processor 12, the signal compressor 123 receives an amplitude envelope of the t-th clean frame x_(t) within the specific frequency band from the noise reduction unit 126 through the band-pass filter 121 and the envelope detection unit 122. The amplitude envelope 30 of the t-th clean frame x_(t) is illustrated in FIG. 6. As shown in FIG. 6, the amplitude envelope 30 of t-th clean frame x_(t) is time-varying.

The signal compressor 123 of the present invention comprises a compression unit 127, a boundary calculation unit 128, and a compression-factor-providing unit 129. The compression unit 127 and the boundary calculation unit 128 are connected to the envelope detection unit 122 to receive the amplitude envelope 30 of the t-th clean frame x_(t), real-time. With reference to FIGS. 5 and 6, the boundary calculation unit 128 can detect an upper boundary UB and a lower boundary LB in the amplitude envelope of the t-th clean frame x_(t). The calculation result of the upper boundary UB and the lower boundary LB are transmitted to the compression-factor-providing unit 129. The upper boundary UB and the lower boundary LB can be calculated by:

UB=x _(t)+α₀×(max(x _(t))− x _(t))  (7)

LB=x _(t)+α₀×(min(x _(t))− x _(t))  (8)

where α₀ is an initial value.

The compression unit 127 receives the amplitude envelope 30 of the t-th clean frame x_(t) and outputs a t-th output frame z_(t). Inputs of the compression-factor-providing unit 129 is connected to an input of the compression unit 127, an output of the compression unit 127, and an output of the boundary calculation unit 128 to receive a calculating result of the upper boundary UB, the lower boundary LB, and the t-th output frame z_(t). An output of the compression-factor-providing unit 129 is connected to the input of the compression unit 127, such that the compression-factor-providing unit 129 provides a compression factor α_(t) to the compression unit 127. The compression factor α_(t) is determined according to a previous compression factor α_(t-1), the upper boundary UB, the lower boundary LB, and the t-th output frame z_(t). When the t-th output frame z_(t) is in a monitoring range between the upper boundary UB and the lower boundary LB, the compression factor α_(t) is expressed as:

α_(t)=α_(t-1)+Δα₁  (9)

where Δα₁ is a positive value (i.e., Δα₁=1).

In contrast, when the t-th output frame z_(t) is beyond the monitoring range, the compression factor α_(t) is expressed as:

α_(t)=α_(t-1)+Δα₂  (10)

where Δα₂ is a negative value (i.e., Δα₂=−0.1).

The t-th output frame z_(t) can be expressed as:

z _(t)=α_(t)×(x _(t) −x _(t))+ x _(t)  (11)

where x _(t) is a mean of the amplitude envelope of the t-th clean frame x_(t).

According to equation (11), the t-th output frame z_(t) is repeatedly adjusted by the t-th clean frame x_(t) and the calculation result of UB, LB, and α_(t). According to experimental result, the signal compression capability is good. As illustrated in FIG. 7, speech components A in the t-th output frame z_(t) are amplified. The speech components A even reach the upper boundary UB. In contrast, noise components B are not exactly amplified. Therefore, the t-th output frame z_(t) is enhanced to stimulate the cochlear nerves. The user can accurately understand a conversation. 

1. A signal processing method in cochlear implant, the signal processing method performed by a speech processor and comprising a noise reduction stage and a signal compression stage, the noise reduction stage comprising: receiving a t-th noisy frame y_(t), wherein the t-th noisy frame y_(t) is from an electrical speech signal y; reducing noises in the t-th noisy frame y_(t) to obtain a t-th clean frame x_(t); and outputting the t-th clean frame x_(t); and the signal compression stage comprising: receiving an amplitude envelope of the t-th clean frame x_(t); compressing the t-th clean frame x_(t) to form a t-th output frame z _(t)=α_(t)×(x _(t) −x _(t))+ x _(t), wherein x _(t) is a mean of the amplitude envelope of the t-th clean frame x_(t); α_(t) is a compression factor; when the t-th output frame z_(t) is in a monitoring range between an upper boundary and a lower boundary, α_(t)=α_(t-1)+Δα₁ and Δα₁ is a positive value; and when the t-th output frame z_(t) is beyond the monitoring range, α_(t)=α_(t-1)+Δα₂ and Δα₂ is a negative value; and outputting the t-th output frame z_(t).
 2. The signal processing method as claimed in claim 1, wherein the t-th clean frame x_(t) in the noise reduction stage is expressed as x _(t) =W ₂ h(y _(t))+b ₂ where h(y_(t)) is a function including W₁ and b₁ in time domain; W₁ and W₂ are default connection weights in time domain; and b₁ and b₂ are default vectors of biases of hidden layers of a DDAE (deep denoising autoencoder)-based NR (noise reduction) structure in time domain.
 3. The signal processing method as claimed in claim 1, wherein the t-th clean frame x_(t), in the noise reduction stage is expressed as x _(t) =InvF{(W ₂ ′h′(F{y _(t)})+b ₂′)} where F { } is a Fourier transform function to transfer the t-th noisy frame y_(t) from time domain to frequency domain; h′ ( ) is a function including W₁′ and b₁′; W₁′ and W₂′ are default connection weights in frequency domain; b₁′ and b₂′ are default vectors of biases of hidden layers of a DDAE-based NR structure in frequency domain; and InvF { } is an inverse Fourier transform function to obtain the t-th clean frame x_(t).
 4. The signal processing method as claimed in claim 2, wherein h(y_(t)) in the noise reduction stage is expressed as ${h\left( y_{t} \right)} = {\frac{1}{1 + {\exp \left\lbrack {- \left( {{W_{1}y_{t}} + b_{1}} \right)} \right\rbrack}}.}$
 5. The signal processing method as claimed in claim 3, wherein h′ (F {y_(t)}) in the noise reduction stage is expressed as ${h^{\prime}\left( {F\left\{ y_{t} \right\}} \right)} = {\frac{1}{1 + {\exp \left\lbrack {- \left( {{W_{1}^{\prime}F\left\{ y_{t} \right\}} + b_{1}^{\prime}} \right)} \right\rbrack}}.}$
 6. The signal processing method as claimed in claim 1, wherein the upper boundary in the signal compression stage is expressed as: UB=x _(t)+α₀×(max(x _(t))− x _(t)); and the lower boundary in the signal compression stage is expressed as: LB=x _(t)+α₀×(min(x _(t))− x _(t)).
 7. The signal processing method as claimed in claim 2, wherein the upper boundary in the signal compression stage is expressed as: UB=x _(t)+α₀×(max(x _(t))− x _(t)); and the lower boundary in the signal compression stage is expressed as: LB=x _(t)+α₀×(min(x _(t))− x _(t)).
 8. The signal processing method as claimed in claim 3, wherein the upper boundary in the signal compression stage is expressed as: UB=x _(t)+α₀×(max(x _(t))− x _(t)); and the lower boundary in the signal compression stage is expressed as: LB=x _(t)+α₀×(min(x _(t))− x _(t)).
 9. The signal processing method as claimed in claim 4, wherein the upper boundary in the signal compression stage is expressed as: UB=x _(t)+α₀×(max(x _(t))− x _(t)); and the lower boundary in the signal compression stage is expressed as: LB=x _(t)+α₀×(min(x _(t))− x _(t)).
 10. The signal processing method as claimed in claim 5, wherein the upper boundary in the signal compression stage is expressed as: UB=x _(t)+α₀×(max(x _(t))− x _(t)); and the lower boundary in the signal compression stage is expressed as: LB=x _(t)+α₀×(min(x _(t))− x _(t)).
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 21. A signal processing method for computationally compressing an input speech signal into a predetermined amplitude range performed at a processing unit of a cochlear implant, comprising: receiving a plurality of time-sequenced input speech signal frames; reducing noise in said plurality of time-sequenced input speech signal frames; determining a compression factor for a first noise-reduced input speech signal frame based on an upper boundary and a lower boundary of the signal amplitude for the first frame; modifying the compression factor for the next noise reduced input speech signal frame based on the next frame's upper boundary and lower boundary of its signal amplitude; and outputting the compressed frames of the time-sequenced input speech signal frames. 